Wireless communication systems are well known in the art. Generally, such systems comprise communication stations, which transmit and receive wireless communication signals between each other. Depending upon the type of system, communication stations typically are one of two types of wireless transmit/receive units (WTRUs): one type is the base station, the other is the subscriber unit, which may be mobile.
The term base station as used herein includes, but is not limited to, a base station, access point, Node B, site controller, or other interfacing device or WTRU in a wireless environment, that provides other WTRUs with wireless access to a network with which the AP is associated.
The term wireless transmit/receive units (WTRU) as used herein includes, but is not limited to, a user equipment, mobile station, fixed or mobile subscriber unit, pager, or any other type of device capable of operating in a wireless environment. Such WTRUs include personal communication devices, such as phones, video phones, and Internet ready phones that have network connections. In addition, WTRUs include portable personal computing devices, such as PDAs and notebook computers with wireless modems that have similar network capabilities. WTRUs that are portable or can otherwise change location are referred to as mobile units.
Typically, a network of base stations is provided wherein each base station is capable of conducting concurrent wireless communications with appropriately configured WTRUs, as well as multiple appropriately configured base stations. Some WTRUs may alternatively be configured to conduct wireless communications directly between each other, i.e., without being relayed through a network via a base station. This is commonly called peer-to-peer wireless communications. Where a WTRU is configured to communicate directly with other WTRUs it may itself also be configured as and function as a base station. WTRUs can be configured for use in multiple networks, with both network and peer-to-peer communications capabilities.
One type of wireless system, called a wireless local area network (WLAN), can be configured to conduct wireless communications with WTRUs equipped with WLAN modems that are also able to conduct peer-to-peer communications with similarly equipped WTRUs. Currently, WLAN modems are being integrated into many traditional communicating and computing devices by manufacturers. For example, cellular phones, personal digital assistants, and laptop computers are being built with one or more WLAN modems.
Popular WLAN environments with one or more WLAN base stations, typically called access points (APs), include those constructed according to one or more of the IEEE 802 family of standards. Access to these networks usually requires user authentication procedures. Protocols for such systems are presently being standardized in the WLAN technology area. One such framework of protocols is represented by the IEEE 802 family of standards.
A basic service set (BSS) is the basic building block of an IEEE 802.11 WLAN, which comprises WTRUs also referred to as stations (STAs). A set of STAs which can talk to each other can form a BSS. Multiple BSSs are interconnected through an architectural component called a distribution system (DS), to form an extended service set (ESS). An access point (AP) is a WTRU that provides access to the DS by providing DS services, and generally allows concurrent access to the DS by multiple STAs.
A network of WTRUs operating with peer to peer communications in an IEEE 802.11 environment, typically referred to as “ad hoc” mode, is also called an “independent BSS.” In an independent BSS, two or more WTRUs establish communication among themselves without the need of a coordinating network element. No AP-to-network infrastructure is required. However, an AP can be configured to use the ad hoc protocols and act as the WTRUs do in peer to peer communications. In such case an AP may act as a bridge or router to another network or to the Internet.
A WTRU that starts an ad hoc network selects the ad hoc network's operating parameters, such as the service set identifier (SSID), channel, and beacon timing, and transmits this information in communication frames, for example, in beacon frames. As other WTRUs join the ad hoc network, they detect and use the ad hoc network's operating parameters.
Where a network infrastructure is used and wireless communications are controlled through APs, parameters such as the SSID are normally specified by a network controller associated with the APs. The APs periodically broadcast beacon frames to enable WTRUs to identify the APs and attempt to establish communications with them. For example in FIG. 1, a WLAN is illustrated in which WTRUs conduct wireless communications via a network station, in this case an AP. The AP is connected with other network infrastructure such as a Network Management Station (NMS). The AP is shown as conducting communications with five WTRUs. The communications are coordinated and synchronized through the AP. Generally, the WLAN system supports WTRUs with different data rates. In some cases an AP is configured to support multiple types of WTRUs, such as 802.11(b) and 802.11(g) compliant WTRUs.
The SSID in an IEEE 802 based system can be a 32-character unique identifier attached to a header of packets sent over a WLAN. The SSID then acts as a password when a WTRU attempts to connect to a BSS or an independent BSS. The SSID differentiates one WLAN from another, so all base stations and all other devices connected to or attempting to connect to a specific WLAN normally use the same SSID. A device will not normally be permitted to join a BSS unless it can provide the correct SSID.
A WLAN system made in accordance with the IEEE 802.11 standard, typically uses a carrier sensing mechanism, where WTRUs sense a wireless medium, such as a particular communication channel, before transmitting data packets to an AP, and only transmit the data packets when the medium is free. If the medium is busy, a WTRU defers its transmission. This works reasonably well when WTRUs are able to receive transmissions of other WTRUs communicating in the WLAN. However, some WTRUs may be hidden from others, and, accordingly, cannot always detect when the medium is busy.
For example, both a first WTRU and a second WTRU may be positioned where they are each able to communicate with an AP, but, due to their locations, they are not able to communicate with each other. An obstruction between the two WTRUs, such as a building or a mountain, can cause this situation. When the first WTRU transmits to the AP, the second WTRU is not able to sense that the medium is busy due to the obstruction or other cause of lack of communication between the two WTRUs. If the second WTRU begins to transmit at the same time as the first WTRU is, the packets will collide at the AP (i.e., the AP is not able to decode the packets received from both WTRUs at the same time).
In order to reduce the severity or avoid this problem, a Network Allocation Vector (NAV) is conventionally used when transmitting on the medium. The NAV provides a timing function that blocks all WTRUs that receive it from transmitting during a period of time set by the NAV. The WTRUs that receive the NAV assume that the medium is busy during the period of time equal to the NAV time period. After the NAV period, the WTRUs that had previously received the NAV are free to normally contend for the medium. Where request to send (RTS) and clear to send (CTS) signaling is used between a WTRU and an AP to grant WTRU requests to transmit data packets, an NAV will be included with the RTS advising all WTRUs in the range of the WTRU transmitting the RTS not to transmit on the medium. In turn, a CTS response from the AP will include a NAV that expires contemporaneously with the NAV in the RTS, thereby alerting all WTRUs receiving the CTS to defer transmissions until the expiration of the NAV. The WTRU that sent the RTS and receives a responsive CTS transmits during the NAV period since the CTS overrides the NAV in the CTS for that WTRU. A responsive CTS normally will not be sent in response to a WTRU's RTS where that RTS is transmitted during the NAV of a prior RTS received by the AP.
Various types of antenna systems can be employed by WTRUs. A switched beam antenna system is a system where multiple fixed beams are defined and the beam that provides the greatest signal enhancements and interference reduction is usually selected for conducting a communication. By using a directional antenna instead of an omni-directional antenna, a higher signal-to-noise ratio (SNR) may be obtained, allowing the link to operate at higher data rates. For base stations, such as APs, the directional beam can also extend the geographic service area of coverage in the direction of the beam. Thus, a switched beam antenna system may improve the coverage area and transmission speed due to the gains provided by directional beams instead of an omni-directional beam for wireless communications. Using a switched beam antenna system, an AP can select the best beam to be used to transmit and receive, depending on the location of a WTRU which is accessing the AP's network via the AP. The selection can be based on any metric that reflects an improvement in transmission and/or reception of the wireless signals.
Collision problems, such as discussed above, exist irrespective of whether a omni-directional or a switched beam antenna system is employed by an AP. For example, consider the case where a first WTRU and a second WTRU are located at opposite sides of an AP that employs a switched beam antenna system. To send data packets to the first WTRU, the AP activates a beam pointed toward the first WTRU and starts data packet transmission. At this time, the second WTRU is likely not able to detect the AP transmission because the beam is pointing toward the first WTRU. Because the second WTRU is unlikely to sense any data traffic, the second WTRU could start transmitting at the same time that the AP is transmitting. If the first and second WTRUs are not hidden from each other, the first WTRU can receive both the second WTRU's transmissions and the AP's transmission creating a potential collision such that the first WTRU would not receive the AP's transmitted data packets successfully.
In order to avoid this problem, before starting to send the data packets to the first WTRU, the AP can notify all other WTRUs in the AP's service area that the medium will be busy. As explained above, this is typically done using a NAV. For the AP to reach all WTRUs, the AP can use an omni-directional signal to transmit the NAV information. However, if the AP uses an omni-directional signal to communicate with all WTRUs before every transmission, then the coverage area of the switched beam antenna is effectively limited to the area that can be reached by the omni directional antenna. Thus, this solution does not extend the coverage area to the full range of a switched directional antenna system.
“Beam sweeping” has been proposed for switched antenna systems such that the antenna beam changes position with time, serving each respective sector for a period of time. Sectors are visited sequentially, or following some suitable pattern based on the system conditions (system load, users' positions, etc.). Different methods have been proposed for the WTRUs to know when to transmit. For example, synchronization between the WTRU and the AP beam sweeping using GPS geolocation or the like.
A characteristic of WLAN systems is that they use beacon signals, which contain synchronization information necessary for a WTRU to associate with a basic service set (BSS). Beacons are transmitted periodically by the AP to its entire service area, once every beacon period (BP). In a sectored service area, the beacons must periodically be transmitted to every sector within a predetermined time period.
To conserve power, WTRUs may go into a power save mode in between beacons, and only wake up to receive the beacons. If a sector is visited before its beacon period expires, the WTRUs in that sector may be in a power save mode, and any transmissions from the AP will not be received. This situation can be avoided if the beam sweeping pattern is sequential and the beam is redirected at regular intervals. However, regular sequential beam sweeping does not allow for flexible scheduling of the beams and, if the sectors are not equally loaded, performance can deteriorate in the more heavily loaded sectors. A variety of irregular sequences are not feasible when a beacon must be transmitted to every sector at the expiration of every beacon period. For example, in a case where there are seven sectors, 1–7, a sequence such as 1, 2, 3, 4, 2, 5, 6, 2, 7, 1 would not be feasible if the beacon period expired in the time service started for sector 1 and ended for sector 6, since sector 7 would not have had a beacon signal sent within the beacon period.
It is desirable to provide collision avoidance techniques and equipment to implement such techniques for beam sweeping systems that facilitate a high degree of service flexibility for such systems. The inventors have recognized that NAV can be used in a beam sweeping system in order to reduce or eliminate the interference that WTRUs from one sector can cause to WTRUs in another sector.
The inventors have also recognized that there are undesirable consequences associated with a strict NAV collision avoidance approach in a beam sweeping system. Neighboring beams usually have an area of overlap. WTRUs that are located in an overlapping area could potentially be served by 2 beams, improving their throughput. However, once a WTRU receives an NAV in one beam, the WTRU defers transmissions for (N−1)*BP/N, and does not transmit in the neighboring beam. Moreover, when a WTRU first turns on and joins a WLAN, the WTRU may join via the beam that appears first, which might not be the best beam. Also, the signal strength near the boundary of the beam pattern will be attenuated, and WTRUs near the beam boundary will experience reduced data rates. Serving WTRUs by more than one beam is desirable to compensate for the lower data rates.